1. Field of the Invention
The present invention relates to a manufacturing method for an oriented film of polyvinyl alcohol derived films used for a polarizing film etc. Furthermore, the present invention relates to a polarizing film comprising an oriented film obtainable by the manufacturing method for the oriented film concerned. A polarizing film is used as a polarizing plate etc. for a visual display, such as a liquid crystal display, an organic EL display, a PDP (plasma display panel).
2. Description of the Prior Art
Conventionally, an oriented film of polyvinyl alcohol derived films has been used as a polarizing film for a liquid crystal display etc. As a manufacturing method for the oriented film concerned, a wet type stretching method and a dry type stretching method may be mentioned. Since moisture percentage of a film gives some effects to stretching, in the wet type stretching method, a stretch irregularity is easy to be obtained in an oriented film. On the other hand in the dry type stretching method, since stretching is conducted in a way that tensile stress is given, using a ratio of peripheral velocity between stretching rolls, to a film heated to be stretched at no less than a glass transition temperature, some un-uniformity based on a deformation by the tensile stress may be arisen when the film is stretched thinner, and, as a result, a stretch irregularity may easily obtained. In a polarizing film using the oriented film that has the above described stretch irregularity, problems of color irregularity and performance irregularity may possibly occur. Japanese Patent Publication No. 2731813 and Japanese Patent Publication No.1524033, etc. are proposed to problems of the manufacturing method for oriented film applying above described dry type stretching method.
However, in the above described dry type stretching method, un-oriented film of polyvinyl alcohol derived films used as conventional master roll usually have a width of about 400 to 2700 mm. When a width of un-oriented film of the master roll is wider than this range, necking becomes bigger at the time of stretching, and the width of oriented film obtained becomes narrower. Moreover, orientation irregularity in the width direction, and thickness irregularity may be easily generated, and oriented film with high degree of orientation may not be obtained.
An object of the present invention is to provide a manufacturing method for an oriented film of polyvinyl alcohol derived film with high degree of orientation, applying dry type stretching method, in case a master roll film having a wide width is used.
Moreover, another object of the present invention is to provide a polarizing film using an oriented film obtained by the above described manufacturing method and to provide a polarizing plate, and further to provide a visual display using the above described polarizing film or polarizing plate.
As a result of repeated examinations carried out wholeheartedly by the present inventors to solve the above-mentioned problems, a manufacturing method of a polarizing film shown below was found out and the present invention was completed.
The present invention relates to a manufacturing method for an oriented film, comprising steps of: stretching an un-oriented polyvinyl alcohol derived film in dry and in longitudinal uniaxial direction by applying tension produced by peripheral velocity difference between rolls, wherein a value of L/W is not more than 0.6 when a distance between the rolls defined as L and a width of the un-oriented polyvinyl alcohol derived film is defined as W; and further heating at 60 to 160xc2x0 C. after the stretching step.
In the above described present invention in order to control necking, a value of L/W is defined not more than 0.6 according to shorten L, where a distance between rolls, which produce tension for stretching an un-oriented film of polyvinyl alcohol derived film as master roll, is defined as L and a width of the un-oriented film as master roll is defined as W. Thus necking at the stretching step is suppressed. When the value of L/W is larger than 0.6, necking become bigger at the stretching step, and as a result a tendency for the width of oriented film obtained to become narrower. It is preferable to set the distance between rolls further shorter so that the above described value L/W may become not less than 0.2. On the other hand, when the above described value of L/W is set not more than 0.6 and the distance L between rolls is set shorter, an un-oriented film will be stretched in a state with a little necking. However, uniaxial property of the oriented film obtained falls, and orientation falls to generate orientation irregularity in a width direction and thickness irregularity. Therefore, in the present invention, orientation is improved by further applying heat treatment after the above described stretching step. Thus when the width of master roll film becomes wider width, oriented film with highly orientation may be obtained in a way that heat treatment is applied after stretching, while limiting the above described value of L/W and shortening a distance between rolls.
In the above described manufacturing method for an oriented film, it is preferable that the heating step after the stretching step is conducted by heating roll. Heat treatment may be efficiently conducted by heating roll.
In the above described manufacturing method for an oriented film, un-oriented film may have been dyed with iodine or dichroism dyestuff.
Moreover, in the above described manufacturing method for an oriented film, after the un-oriented film is stretched and further applied heat treatment, the oriented film may be dyed with iodine or dichroism dyestuff.
And particularly the present invention relates to a polarizing film comprising an oriented film obtainable by the manufacturing method for the above described oriented film.
And more particularly the present invention relates to a polarizing plate with which an optical transparent protective layer is prepared at least in one side of the above described polarizing film.
Furthermore, the present invention relates to a visual display using the above-described polarizing plate.